1. Field of the Invention
The present invention relates to process safety equipment for hydrocarbon production and in particular, but not exclusively, to a method and apparatus for use with existing well test equipment on temporary process installations.
2. Description of the Related Art
In conventional well testing a considerable amount of equipment is transported to an oil rig and a well test/process situation is set up to test the fluid from the downhole reservoir. A typical prior art test system is shown in FIG. 1 where a number of well test components such as a steam exchanger, a test separator and a surge tank are provided as part of the well test equipment. As each component has a different pressure rating--it is important to monitor the pressure in each of the components such that if an over-pressure situation occurs at any piece of equipment, a safety valve is actuated which vents the over-pressure fluid to atmosphere via the rig relief burner boom. With the system shown in FIG. 1 separate safety valves are coupled to each component. If there are a large number of components this will require an equally large number of safety valves and the monitoring and coupling of such valves is a disadvantage in a rig environment. In addition, in existing well test systems certain parts of the system are ignored and it is assumed that safety valves are not required to be coupled thereto, for example the coil of the heat exchanger. In addition, the safety valves are of the same design and are representative of the prior art.
A common safety valve used is the SPM emergency relief valve which is a spring-operated device using a hardened ball and seat sealing area. The ball is held fast against the seat by valve springs and remains seated until upstream pressure equals the set pressure. At this point the ball begins to unseat to allow liquid to relieve. As upstream pressure increases, the ball compresses a spring and travels away from the seat until an equilibrium is met that allows a given amount of liquid to pass at a pressure above the valve setting. When the pressure drops below the set pressure, the valve reseats. These existing valves are primarily designed to vent liquid and they are not designed to vent multi-phase fluids, such as that in a hydrocarbon production line which is generally a fluid/gas mixture. In addition, these valves do not lock open and are designed for venting relatively low volume. When there is a liquid gas mixture combination and the fluid is at high pressure, the depressurisation curve is very steep so that when the valve initially opens the throttling effect causes the temperature of the gas to fall to such a level that the fluid freezes up and venting does not occur. In this situation the pressure is retained and the well test equipment is then likely to fail at the next weakest point which is probably the piece of the equipment which the valve is intended to protect. In general the downstream side of each piece of equipment is not rated to an equal pressure as the upstream system and may rupture. In addition, these valves are not particularly accurate in pressure rating because at the start the gas temperature may be -40.degree. F. and this temperature can change to +250.degree. F. within half an hour to an hour of start up. The valves are not repeatable and the valve operating point changes because of thermal stress so that venting is or will be unpredictable.
A further problem with the prior art arrangement is that there is no in-line block valve which means that pressure tests can only be carried out at a value less than the safety valve threshold with the result that the valve rating for a fully open position cannot be checked. With the arrangement shown in FIG. 1 only the part of the well test equipment to which the valve is attached is protected and, consequently, many safety valves are required, for example in FIG. 1, six safety valves SV.sub.1 -SV.sub.6, are shown and this only provides partial protection for the system.
It is an object of the present invention to provide a well test system in which the requirement of multi-safety valves is obviated and which allows pressure testing to be carried out at and above the pressure to the full value of the production line portion in which the valve is situated.
Another object of the present invention is to provide a relief valve which obviates or mitigates at least of the aforementioned disadvantages.